91 - HOW TO GET TROUBLE CODES

1991 Buick Lesabre

I have tried looking for information on how to get the trouble codes for my daughters 91 lesabre. If someone would let me know at least where under the dash to get started and any information on how I would REALLY appreciate it. She is getting " service engine soon " - Haynes manual is no help what-so-ever (argggh) -
81,000 miles V6
Thanks -